Heart of Darkness is one of those famous (or notorious) books that turn out to be very different from your preconceptions. As most of you probably know, Heart of Darkness is the story of Marlow, a sailor who journeys up the River Congo to the heart of Africa to rescue Kurtz, an ivory-raider who has "gone native".
I really enjoyed it, much more than I was expecting. The prose is so lyrical and atmospheric that you feel as if you really are in the deep, menacing jungle, unknown and unknowable. The story is unsettling and unresolved; there is no clear "answer" to the questions Conrad raises, about imperialism, death and the darkness of the heart. Don't rely on what others tell you about it - read it for yourself; it's a classic that needs to be read.
(Don't worry, it's nice and short - 78 pages in this edition. Although there are parts that a squeamish reader might want to skip over.)